Ray Radebaugh

Cryocoolers: the state of the art and recent developments

Cryocooler performance and reliability are continually improving. Consequently, they are more and more frequently implemented by physicists in their laboratory experiments or for commercial and space applications. The five kinds of cryocoolers most commonly used to provide cryogenic temperatures for various applications are the Joule-Thomson, Brayton, Stirling, Gifford-McMahon, and pulse tube cryocoolers. Many advances in all types have occurred in the past 20 years that have allowed all of them to be used for a wide variety of applications. The present state of the art and on-going developments of these cryocoolers are reviewed in this paper. In the past five years new research on these cryocoolers has offered the potential to significantly improve them and make them suitable for even more applications. The general trend of this new cryocooler research is also presented.

A Review of Pulse Tube Refrigeration

This paper reviews the development of the three types of pulse tube refrigerators: basic, resonant, and orifice types. The principles of operation are given. It is shown that the pulse tube refrigerator is a variation of the Stirling-cycle refrigerator, where the moving displacer is substituted by a heat transfer mechanism or by an orifice to bring about the proper phase shifts between pressure and mass flow rate. A harmonic analysis with phasors is described which gives reasonable results for the refrigeration power yet is simple enough to make clear the processes which give rise to the refrigeration. The efficiency and refrigeration power are compared with those of other refrigeration cycles. A brief review is given of the research being done at various laboratories on both one and two-stage pulse tubes. A preliminary assessment of the role of pulse tube refrigerators is discussed.

Cryogenic Material Properties Database

NIST has published at least two references compiling cryogenic material properties. These include the Handbook on Materials for Superconducting Machinery and the LNG Materials & Fluids. Neither has been updated since 1977 and are currently out of print. While there is a great deal of published data on cryogenic material properties, it is often difficult to find and not in a form that is convenient to use. We have begun a new program to collect, compile, and correlate property information for materials used in cryogenics. The initial phase of this program has focused on picking simple models to use for thermal conductivity, thermal expansion, and specific heat. We have broken down the temperature scale into four ranges: a) less than 4 K, b) 4 K to 77 K, c) 77 K to 300 K, and d) 300 K to the melting point. Initial materials that we have compiled include oxygen free copper, 6061-T6 aluminum, G-10 fiberglass epoxy, 718 Inconel, Kevlar, niobium titanium (NbTi), beryllium copper, polyamide (nylon), polyimide, 304 stainless steel, Teflon, and Ti-6Al-4V titanium alloy. Correlations are given for each material and property over some of the temperature range. We will continue to add new materials and increase the temperature range. We hope to offer these material properties as subroutines that can be called from your own code or from within commercial software packages. We will also identify where new measurements need to be made to give complete property prediction from 50 mK to the melting point.

A comparison of three types of pulse tube refrigerators: new methods for reaching 60K

Pulse tube or thermoacoustic refrigerators require only one moving part—an oscillating piston or diaphragm at room temperature. Refrigeration occurs within a tube connected to the pressure wave generator when the thermal relaxation time between gas and tube is comparable to a half period. Three types have been discussed in the literature recently by Gifford, by Mikulin, and by Wheatley. A record low temperature of 60 K was achieved in our work using a single stage pulse tube similar to that of Mikulin. Previously 105 K was the lowest temperature achieved. Because of only one moving part, all three types have the potential for long life, but their efficiency and intrinsic limitations have never been investigated. This paper compares the three types with each other and with common refrigerators such as Joule-Thomson and Stirling refrigerators. An apparatus is described which can measure the intrinsic behavior of the different types from temperatures of about 30 K to 300 K. Overall cycle efficiency as well as sources of loss such as conduction and regenerator ineffectiveness are discussed and the advantages of various phase shifting techniques to increase refrigeration capacity are compared.

Pulse tube cryocoolers for cooling infrared sensors

This paper reviews recent advances in pulse tube cryocoolers and their application for cooling infrared sensors. There are many advantages of pulse tube cryocoolers over Stirling cryocoolers associated with the absence of moving parts in the cold head. Efficiencies have been improved considerably in the last few years to where they equal or even exceed the efficiencies of Stirling cryocoolers. The use of inertance effects and double inlets to improve the efficiencies will be discussed. Pulse tube cryocoolers are now being used or considered for use in cooling infrared detectors for many space applications. One disadvantage of pulse tube coolers is the difficulty in scaling them down to sizes as small as 0.15 W at 80K while maintaining high efficiency. A second disadvantage is the larger diameter cold finger required for the same refrigeration power because of the presence of the pulse tube. These two disadvantages have limited their use so far in cooling infrared sensors for many military tactical applications. Progress in overcoming these disadvantages is discussed.

Development and Experimental Test of an Analytical Model of the Orifice Pulse Tube Refrigerator

The promise of high reliability and high refrigeration capacity is responsible for a recent surge of interest in pulse tube refrigeration. This work involves the development of an analytical model describing behavior of the orifice pulse tube to gain a better understanding of the refrigeration process. Due to oscillating gas flow, the system is described in terms of average enthalpy flow with such simplifying assumptions as an ideal gas and sinusoidal pressure variation. Phasor analysis is used to represent the temperature, pressure, and mass flow rate waves in vector form. Also discussed in this paper is the verification of the model in which analytical predictions are compared to experimental measurements. The results confirm predictions by the model that refrigeration power is proportional to the average pressure, the pulse frequency, the mass flow ratio, and the square of the dynamic pressure ratio. Also, a temperature probe was devised to measure the average temperature profile and dynamic temperature in the tube. As a result of simplifying assumptions, magnitudes of refrigeration power from the model are between 3 and 5 times greater than experimental values.

Characterization of 350 Hz thermoacoustic driven orifice pulse tube refrigerator with measurements of the phase of the mass flow and pressure

The world’s first 350 Hz thermoacoustic driven orifice pulse tube refrigerator (TADOPTR) has been designed and built by Tektronix, Inc., in cooperation with Los Alamos National Laboratories (LANL) and the National Institute of Standards and Technology (NIST). This highly instrumented system includes hot wire anemometers and pressure sensors for measuring the phase of the mass flow and pressure at all key locations in the TADOPTR, permitting for the first time detailed comparison to analytical models developed by LANL and NIST. Characterization results for velocity and pressure phase, pressure amplitude, and enthalpy flow show good agreement with the simulations. We have also demonstrated a new design method that uses the inertance of the pulse tube at 350 Hz to achieve the desired phase between the mass flow and pressure, rather than the usual double inlet design. We have designed and characterized single stage and two stage 350 Hz TADOPTRs.

Feasibility of electrocaloric refrigeration for the 4–15 K temperature range☆

Abstract The feasibility of a solid state type of refrigeration, which utilizes the electrocaloric effect in certain dielectric materials, has been investigated. The study was limited to the temperature range where the refrigerator would absorb heat from a load at about 4 K and reject heat to a reservoir at about 15 K. Heat switches would be required for such a refrigerator and two types were studied. One type was a multiple-leaf contact switch, the other a magnetothermal switch of single crystal beryllium. Many electrocaloric materials were studied but none was found with a sufficiently large reversible electrocaloric effect for a practical refrigerator. The largest effects were seen in a SrTiO 3 ceramic, followed by a KTaO 3 single crystal. Temperature reductions of about 0.3 K at 10 K were observed during depolarization from fields of 20 kV cm −1 . A theoretical model, based on the electret behaviour of impurity-vacancy dipoles is postulated to interpret the anomalous dielectric behaviour of the materials investigated. Another theoretical model, based on the lattice dynamics of displacive dielectrics, is postulated to explain the observed temperature changes seen in such materials. The model points out that at 4 K the entropies of displacive type materials are probably too low for practical refrigeration. An investigation of certain order-disorder dielectrics is suggested.

Inertance Tube Optimization for Pulse Tube Refrigerators

The efficiency of regenerative refrigerators is generally maximized when the pressure and flow are in phase near the midpoint of the regenerator. Such a phase relationship minimizes the amplitude of the mass flow for a given acoustic power flow through the regenerator. To achieve this phase relationship in a pulse tube refrigerator requires that the flow at the warm end of the pulse tube lag the pressure by about 60 degrees. The inertance tube allows for the flow to lag the pressure, but such a large phase shift is only possible with relatively large acoustic power flows. In small pulse tube cryocoolers the efficiency is improved by maximizing the phase shift in the inertance tube. This paper describes a simple transmission line model of the inertance tube, which is used to find the maximum phase shift and the corresponding diameter and length of the optimized inertance tube. Acoustic power flows between 1 and 100 W are considered in this study, though the model may be valid for larger systems as well. Fo...

120 Hz pulse tube cryocooler for fast cooldown to 50 K

A pulse tube cryocooler operating at 120 Hz with 3.5 MPa average pressure achieved a no-load temperature of about 49.9 K and a cooldown time to 80 K of 5.5 min. The net refrigeration power at 80 K was 3.35 W with an efficiency of 19.7% of Carnot when referred to input pressure-volume (PV or acoustic) power. Such low temperatures have not been previously achieved for operating frequencies above 100 Hz. The high frequency operation leads to reduced cryocooler volume for a given refrigeration power, which is important to many applications and can enable development of microcryocoolers for microelectromechanical system applications